In an optical disc player for optically recording an information signal on or optically reproducing an information signal from an optical disc-shaped record medium, an optical head device is provided for constituting an optical arrangement for causing a light beam to impinge upon the optical disc-shaped record medium so as to form a spiral record track corresponding to a recording information signal on the optical disc-shaped record medium or to read an information signal recorded in the spiral record track on the optical disc-shaped record medium.
In the optical head device, for example, a light beam produced by a light beam generating portion is collimated by a collimator lens and enters into an objective lens to be focused thereby to impinge upon the optical disc-shaped record medium, and then a reflected light beam emanating from the optical disc-shaped record medium is directed through the objective lens to a beam splitter to be changed in a direction of its optical axis thereby so as to enter into a photodetecting portion. A reproduced information signal, a focus error signal and a tracking error signal are produced based on a detection output of the reflected light beam from the photodetecting portion, and a focus servocontrol operation for maintain correct focus of the light beam projected onto the optical disc-shaped record medium and a tracking servocontrol operation for maintain the light beam in correct tracking relation to each turn of the spiral record track on the optical disc-shaped record medium are performed in accordance with the focus error signal and the tracking error signal, respectively.
As for the production of the tracking error signal, various signal producing systems including the so-called "Push-Pull system" have been known. In the Push-Pull system, the reflected light beam from the optical disc-shaped record medium is detected by a photosensor having a photodetecting element divided into two parts, and the tracking error signal is produced based on a difference in level between detection outputs obtained from two parts of the photodetecting element, respectively. In the case where the tracking error signal is produced in accordance with the Push-Pull system, although an optical arrangement for obtaining the tracking error signal and an associated signal processing circuit arrangement can be relatively simple, there is a disadvantage that a light beam spot formed on the photosensor by the reflected light beam from the optical disc-shaped record medium is undesirably moved regardless of the tracking condition of the light beam impinging upon the optical disc-shaped record medium, and therefore the tracking error signal contains undesirable DC offsets when the optical disc-shaped record medium is inclined in its radial direction to the optical axis of the objective lens through which the light beam is incident upon the optical disc-shaped record medium.
Accordingly, with the intention of avoiding the above mentioned disadvantage of the Push-Pull system, there has been proposed an improved Push-Pull system as disclosed in the Japanese patent application published before examination with laid-open number 61-94246. In this case, at least two light beams which are obtained by dividing a light beam produced by a light beam source are caused to impinge upon an optical disc-shaped record medium for forming respective light spots thereon with a space therebetween in the radial direction of the optical disc-shaped record medium, which corresponds to a distance in a predetermined relation to the track pitch of the spiral record track. Then, each of two reflected light beams from the optical disc-shaped record medium is detected by a respective photosensor having a photodetecting element divided into two parts, and a difference in level between detection outputs obtained respectively from the two parts of one photodetecting element is subjected to level adjustment for compensating a difference in strength between the two reflected light beams and then subtracted from a difference in level between detection outputs obtained respectively from the two parts of the other photodetecting element, thereby to produce the tracking error signal.
The tracking error signal thus obtained is prevented from containing DC offsets, so as properly to represent the tracking condition of the light beam impinging upon the optical disc-shaped record medium, even if it is inclined in its radial direction to the optical axis of the objective lens.
It is not seldom that the optical disc-shaped record medium on which the light beam is focused by the objective lens to impinge is provided thereon with a guide groove preformed for indicating a position where the spiral record track is to be made and further it is usual that the spiral record track comprises a plurality of pits or bumps provided to align on the optical disc-shaped record medium.
Accordingly, when the spiral record track corresponding to the recording information signal is formed to extend along the guide groove preformed on the optical disc-shaped record medium by the light beam impinging thereon or the information signal is read from the spiral record track by the light beam impinging on the optical disc-shaped record medium to scan the spiral record track, a tracking error signal varies in amplitude in response to variations in depth of the guide groove, variations in depth of each of the pits or in height of each of the bumps forming the spiral record track, variations in reflectance to a light beam of the optical disc-shaped record medium, and variations in quantity of the light beam impinging on the optical disc-shaped record medium even if it is produced in accordance with the improved Push-Pull system. In the previously proposed optical disc player, such variations in amplitude of the tracking error signal exert disadvantagious influences on the tracking servocontrol for the light beam impinging on the optical disc-shaped record medium to form the spiral record track thereon or to scan the spiral record track formed thereon.